A motor vehicle seat is provided with a backrest and a seat portion having a fixed element and at least one body which can be moved between a lowered configuration, in which it forms with the backrest a place for an adult passenger, and a raised configuration, in which it forms a booster seat suitable for supporting a child; the body is releasably fixed in the raised configuration by means of a fastening device and carries a side support element, which is arranged next to the body and is movable between an operating configuration, in which it protrudes upwards with respect to an upper surface of the body, and a rest configuration, in which it is arranged lower or flush with respect to said upper surface.